1. Field of the Invention:
The present invention relates to a memory device to be mounted on, for example, an IC card, and in particular, to a memory device in which data writing is prohibited by means of hardware and a method for prohibiting writing to such a memory device.
2. Description of the Related Art:
In a conventional memory device, such as conventional EEPROM (electrically erasable programmable read only memory), erroneous writing caused by misoperation when an apparatus in which the EEPROM is used is operated at a low voltage is prevented by detecting the voltage applied to the EEPROM and prohibiting writing until the voltage reaches a normal operation level. In such a system configuration, in other words, writing to the EEPROM is prohibited by means of hardware when the operating voltage is low, but no such prohibition is performed when the operating voltage is high.
An IC card, on which this type of EEPROM is mounted, requires a high degree of security. Accordingly, a conventional IC card has a configuration in which writing to an EEPROM is prohibited by means of software when the IC card is operated at a normal voltage. Namely, writing prohibition which is usually controlled by application software is controlled by programming software in such an IC card.
More partically, in the case when the validity of the IC card has expired, writing to an EEPROM is prohibited by programming software based on data stored in a built-in timer or data concerning dates sent from an external terminal, or rewriting of the registration of an issuer or the like is prohibited by means of software once such registration is made.
However, writing prohibition to an EEPROM only by programming software has a limit since writing to an EEPROM becomes possible when writing is commanded by runaway of the program or when the program is revised. Accordingly, a conventional memory device such an IC card including this type of EEPROM has a limit in improving the security thereof.